1. Field of the Invention
The present invention relates to a shutter blades assembly suitable for an electromagnetically driven shutter.
2. Description of the Prior Art
Generally speaking in case of the electromagnetically driven shutter the electromagnetically force is used as the driving force for the shutter blade, whereby the electromagnetic force is represented by F=BIL and is proportional to the magnetic flux density B, the current I and the length of the coil L.
Consequently in order to drive the shutter blades with a large driving force it is necessary that at least one of the magnetic flux density, the current or the length of the coil should be large. However, the magnetic flux density obtainable from the magnetic material now in use is limited, while the current obtainable from the battery to be loaded in camera is also limited. Further, considering the fact that the coil is built in the camera, the length of the coil cannot be made so long, whereby especially in case of the moving coil type electromagnetically driven shutter the increase of the coil length means an increase of the moment of inertia of the coil itself, which does not always improve the motion characteristics of the moving coil.
Consequently, only a certain limited electromagnetic force can be utilized in order to drive the shutter blades.
On the other hand, the blade for the lens shutter consists of a thin metal plate, while the shutter consists of one, two or more blades, whereby the larger the number of shutter blades, the better the shape of the shutter opening but the more complicated the construction resulting in a larger moment of inertia. Consequently, most of the shutters comprise about five blades as is shown in FIG. 1. The shutter in FIG. 1 consists of five blades 1.sub.1, 1.sub.2, 1.sub.3, 1.sub.4 and 1.sub.5, so designed that there occurs a shutter blade overlap over the adjacent blades when the shutter is closed. The blades 1.sub.1, 1.sub.2, 1.sub.3, 1.sub.4 and 1.sub.5 open along with the rotation of the sector ring 2 in the counterclockwise direction, and they are closed when the sector ring 2 resumes the initial position.
When the shutter constructed as mentioned above is driven by means of the above-mentioned electromagnetic force, it is impossible to drive the shutter at a sufficient speed by means of a certain limited electromagnetic force because the moment of inertia of the shutter blades consisting of thin metal plates is large.
Thus a recent approach has involved replacement of the thin metal plates for the shutter blades with blades consisting of plastics such as polyester so as to reduce the moment of inertia of the movable parts. The specific weight of the plastic is generally 1/6-1/7 as large as that of metals thereby contributing to the reduction of the moment of inertia of the movable parts of the electromagnetically driven shutter.
Consequently, significant improvement in the motion characteristics of the shutter occurs by adoption of plastic shutter blades for the electromagnetically driven shutter.
However, when the plastic shutter blades are constructed of a thickness similar to that of the metal blades, it is impossible to obtain a sufficient light shading ability. The plastic material is naturally colored so as to provide a given a light shading ability, and if the plastic is too thin it permits the passage of some light. When for this reason thicker plastic shutter blades are provided, the moment of inertia of the moving part cannot be reduced, thus eliminating the advantages of plastic shutter blades whereby if thinner shutter blades are used, it is impossible to completely shade the light. Further, it is difficult to build large shutter blades in the camera.
Further, because the plastic shutter blades are arranged in direct contact with the metal plate for fixing the shutter on the camera, when the metal plate is warmed with the heat radiated from the sun, the shutter is also heated in such a manner that the metal plate and the shutter blades often adhere to each other, and the blades are easily deformed. Further, static electricity is produced between the shutter blades and the lacquered metal plate by means of the opening and closing operation of the shutter, which promotes adhesion until at last the shutter blades do not move.